Silicon nitride ceramics are the preferred material for components in advanced heat engines because of their superior mechanical strength, especially at high temperatures, excellent thermal shock resistance and good oxidation resistance. The utilization of the superior performance inherent in silicon nitride ceramics as well as their application in a broad range of vehicular and industrial products is predicated upon the availability of reasonably priced silicon nitride powders with consistent and reproducible properties.
The three major industrially used techniques for the synthesis of silicon nitride are:
1. The reaction between silicon and nitrogen, the so called nitridation of silicon;
2. The reaction between silicon tetrachloride (or some halogenated silane compound) and ammonia;
3. The reaction between silicon dioxide and nitrogen in the presence of carbon, the so called carbothermic reduction of silicon dioxide.
The nitridation of silicon is kinetically a slow process and metallic catalysts, typically iron, are added to the system to produce useful yields. This necessitates subsequent treatment of the product to remove the objectionable metallic impurities. Powders produced by this technique are difficult to sinter to maximum density and are not suitable for the most rigorous structural ceramics applications.
The vapor phase reaction between silicon tetrachloride and ammonia produces, in addition to silicon nitride, a variety of by-products which require expensive processing for their removal. In addition, the synthesis process is difficult to control and all too often gives inconsistent results. Good powders prepared by this method have been used successfully in a variety of structural ceramics. However, the production costs for this material are too high for wide industrial use.
The carbothermic reduction of silicon dioxide as taught in the existing patent literature, is also unsatisfactory and fraught with difficulties. Thus, U.S. Pat. No. 4,117,095 to Komeya et al. requires the addition of elementary silicon to the reaction mixture of silicon dioxide and carbon. This is undesirable not only because of the cost in that silicon of comparable particle size and purity is more expensive then silicon dioxide but also because any small residue of silicon not converted to silicon nitride would adversely affect the properties of the structural ceramic made from the powder. U.S. Pat. No. 4,122,152 to Mori et al. specifies the use of carbon powder having an oil absorption of no less than 100 ml/100 g which again increases the cost of the powder by using a more expensive reactant (carbons with a high oil absorption are generally more expensive than those with low oil absorption). Another variation, as described in U.S. Pat. No. 4,396,587 to Yamaguchi et al., teaches the use of precursors such as liquid silicic acid for the silicon component and carbonaceous substances for the carbon reductant. This introduces additional expense, both in terms of the reagent cost and the extra processing required. The use of silicic acid, as taught in this patent, is particularly undesirable because it was obtained from water glass, a sodium silicate, by ion exchange. Any sodium residue in the powder, even of the order of 100 ppm, is detrimental to the properties of silicon nitride structural ceramics. Still another patent, U.S. Pat. No. 4,414,190 to Seimiya and Nishida, teaches the use of a silicon dioxide dispersion, the so called white carbon, which is obtained from an alkali or alkali earth metal silicate. This process is objectionable for the same reasons as the preceding one.
This review indicates that the carbothermic reduction process for the preparation of silicon nitride, as taught by these inventions, is inadequate for the costeffective preparation of powders useful in structural ceramics applications. There has been no mention made that any of these powders is sinterable to high density or that the resulting ceramics possess the requisite mechanical strength and oxidation resistance.